[SI-LIST] Re: Series Termination Question
- From: Aubrey_Sparkman@xxxxxxxx
- To: zabinski.patrick@xxxxxxxx, si-list@xxxxxxxxxxxxx,breams@xxxxxxxxxxxxxx
- Date: Thu, 18 Apr 2002 09:03:20 -0500
Bill,
Just make sure the R3 resistor request is for Clock and other
uni-directional signals. If not, there is good reason to have R3 on a
Bi-directional bus; it serves as the source termination when the receiver
becomes the receiver.
Aubrey Sparkman
Signal Integrity
Aubrey_Sparkman@xxxxxxxx
(512) 723-3592
-----Original Message-----
From: Zabinski, Patrick J. [mailto:zabinski.patrick@xxxxxxxx]
Sent: Thursday, April 18, 2002 8:44 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Series Termination Question
Bill,
Assuming the receiver has a high input impedance, I can
see no benefit (other than political) in R3. With
a high input impedance, there should be no current flowing
in R3, so it's useless.
If your customer insists on R3, then make it 0.1 ohms. ;)
Pat
> I'm presently designing a board that will be used by company "X". =
> Basically, this is to be one of our storage routers in a form factor =
> that will fit within their product. In going though the
> schematics to =
> ensure that we're going to be giving them what they want, their =
> regulatory engineer made a request that I can't quite get my mind =
> around. I was hoping that someone could give me some insight
> into this =
> request so I can understand why he is asking for this (i.e.,
> what good =
> does it actually do).
>
> Normally, we use series terminators in this fashion with R
> chosen based =
> on driver and trace impedance (as well as what seems to work in the =
> lab):
> R1 __________
> |\__/\/\/\__| Trace |__|\
> Driver |/ |__________| |/ Receiver
>
>
> But the company "X" regulatory guy is asking for this on
> clock signals:
>
> R2 __________ R3
> |\__/\/\/\__| Trace |__/\/\/\__|\
> Driver |/ |__________| |/ Receiver
>
>
> So, what would be the advantage of using two resistors where
> one seems =
> to do the job? And it seems that, if we went with the second
> circuit, R2 =
> + R3 =3D R1 but what other constraints would make sense? For clock =
> signals, we have 40 MHz, 66 MHz, 85 MHz, and 100 MHz.
>
>
> I'm rather caught between not understanding the utility of
> his request =
> and two basic business axioms:
> * The customer is always right.
> * Give the customer what she needs (not necessarily what she wants).
>
> Thanks in advance for your advice.
>
> _______________________________________
> | |
> | Bill Reams - Sr. Hardware Engineer |
> | 512-928-7201 (direct) |
> | 512-349-0300 (Main) |
> | breams@xxxxxxxxxxxxxx (e-mail) |
> |______________________________________|
>
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